1. Field of the Invention
The present invention relates to a mobile telecommunication apparatus such as a portable telephone and a PHS telephone.
2. Description of the Prior Art
FIG. 18 illustrates a conventional mobile telecommunication apparatus shown in Japanese Laid-Open Patent Publication No. 7-74807, "A Portable Telephone". FIG. 18 includes a body 1 of a telecommunication apparatus, an antenna 2, a transmitter 3, and a receiver 4.
FIG. 19 is a perspective view which illustrates an internal construction of a conventional mobile telecommunication apparatus. FIG. 20 and FIG. 21 are sectional views seen from a line A-B and a line C-D in FIG. 19, respectively.
In FIG. 19-FIG. 21, the mobile telecommunication apparatus includes a printed circuit board 5, a radio circuit 6 formed on the printed circuit board 5, a control circuit 7 formed on the printed circuit board 5, a voice circuit 8, shield cases 9a and 9b surrounding the radio circuit 6, a feeder line 10, a feeding circuit 11, an earth conductor 12 for the radio circuit 6, an earth conductor 13 for the control circuit, an earth conductor 14 formed between the earth conductors 12 and 13, and a battery 15.
The printed circuit board 5 is actually a multi-layered circuit board. Parts constructing the radio circuit 6 and the control circuit 7 are mounted on the printed circuit board 5. The voice circuit 8 is included in the control circuit 7.
The shield cases 9a and 9b are equipped to shield external noise coming into the radio circuit 6 and to prevent the escape of radiation to the outside. However, since the shield construction around the control circuit 7 is complicated, the shield construction is not generally included.
The antenna 2 generally uses a .lambda./4 monopole antenna (.lambda.indicates a wavelength of the frequency being used) or a .lambda./2 monopole antenna. Impedance of a .lambda./4 monopole antenna is easily matched and the structure of the feeding circuit 11 is relatively simple. The radiation characteristic of the .lambda./2 monopole antenna is better than the .lambda./4 monopole antenna although the construction of feeding circuit 11 becomes a little bit complicated due to impedance matching. The earth conductor 12, the earth conductor 13, and the earth conductor 14 arc equipped on the printed circuit board 5. The earth conductor 14 is equipped between the earth conductor 12 and the earth conductor 13, and they are connected electrically.
The operation of the conventional mobile telecommunication apparatus is explained below. During transmitting, a human voice from a transmitter 3 is converted to a voice signal through the voice circuit 8 in the control circuit 7. The voice signal is modulated to a high frequency signal by the radio circuit 6. The modulated high frequency signal is radiated from the antenna 2 via the feeding circuit 11.
On the other hand, during receiving, a high frequency modulated signal which enters the radio circuit 6 via the feeding circuit 11 from the antenna 2 is demodulated to a voice signal through a demodulator circuit. The demodulated voice signal is transduced into an audible human voice by the receiver 4 via the voice circuit 8 in the control circuit 7. Also, the battery 15 supplies power to the radio circuit 6 and the control circuit 7.
FIG. 22 illustrates an example of a longitudinal current distribution on the side of the printed circuit board 5 facing the battery 15 in the conventional mobile telecommunication apparatus using a .lambda./4 monopole antenna as the antenna 2.
In FIG. 22, numerals 16 and 17 indicate current amplitude and phase which distribute along the antenna 2 and the radio circuit 6, respectively. Numerals 18 and 19 indicate current amplitude and phase which distribute along the control circuit 7, respectively.
Current flowing through the earth conductor 14 equipped between the radio circuit 6 and the control circuit 7 is amplified by a parallel plane mode oscillation between the battery 15 and the control circuit 7 that generates a sinewave current 18 having large amplitude.
This current having a sinewave distribution greatly influences the voice circuit 8 in the control circuit 7. This current generates noise in the voice signal which is then transduced by the receiver 4 thereby decreasing the speech quality.
Also, the phase 19 of the current flowing along the control circuit 7 on the printed circuit board 5 is contrary to the phase 17 of current distributing between the antenna 2 and the radio circuit 6. Therefore, the antenna gain of the telecommunication apparatus toward the horizontal direction is extremely attenuated, which decreases the available communication distance.